Calorimetry is a powerful technique that is widely employed to measure the enthalpy (including enthalpy change) of chemical reactions, and the heat capacity and other physical properties of solid-state systems and materials. Calorimetry is a technique widely used by the pharmaceutical industry, biologists and chemists to study the kinetics of biochemical reactions and the reaction of living organisms and tissues to chemicals (e.g., drugs).
Currently, microcalorimeters produced by semiconductor processing are routinely used for heat capacity studies upon thin films and for measurements of the heat of reaction of catalytic processes. Current microcalorimeters have a resolution typically on the order of ˜1 fJ/K, limited by the heat capacity of the calorimeter itself, termed the “addendum,” and by and the sensitivity of thermometry utilized. Unfortunately, the relatively poor resolution of these calorimeters render them unsuitable for many small scale applications. The quest to improve the sensitivity of calorimetry is not simply to improve accuracy but, more importantly, to enable measurements upon nanoscale objects such as epitaxial thin films, nanoparticles and nanoclusters, fullerenes, biological macromolecules, and the chemical processes involving individual molecules or individual cells.
Another shortcoming of current microcalorimeters is their inability to take accurate measurements on small volume samples due to insufficient thermal isolation of the samples from the rest of the device. The most common forms of thermal isolation used for calorimeters are thermally resistive enclosures, air cushions (generated by membranes) and insulating substrates. Such techniques are not readily applicable to samples having a volume on the order of 100 pL. A resistive enclosure for a 100 pL volume has to be made as small as the volume itself. The membranes used by the existing microcalorimeters are themselves too conductive (e.g., thermal conductivity of silicon nitride is 30 W/m K) to provide adequate thermal isolation. Finally, because common insulating materials (e.g., foam) are comprised of air sacs which are as big as the microcalorimeter itself, they can not be used as the substrate.
Thus, a need exists for microcalorimeters that provide low addendum heat capacity and high thermal isolation of reagents.